This invention relates to steam turbines and, more particularly, to an apparatus for improving the effectiveness of transporting preseparator condensate from drain collection tanks to moisture separator reheaters drain tanks.
The usefulness of preseparators in order to remove liquid entrained in steam flow through the exhaust system of a steam turbine is well-known. Some exhaust pipes, known as crossunder lines, which connect the exhaust of a high pressure turbine with one or more moisture separators or moisture separator reheaters in a nuclear power plant are subject to serious erosion damage unless a preseparator is incorporated. For simplicity, the terms "moisture separator" and "moisture separator reheater" will be used in the alternative and the use of only one term is intended to include either or both, it being recognized that some turbine systems may not include moisture separator reheaters. The erosion problem has been fully explained in several publications. Specifically, U.S. Pat. No. 4,527,396 issued to Silvestri and assigned to Westinghouse Corporation, discloses a moisture preseparator for removing erosion-causing entrained liquid from a moisture-laden stream of gas flowing in the exhaust pipes of a steam turbine. That invention is part of a larger scheme to remove liquid entrained in the exhaust steam flowing in the entire steam turbine exhaust system and to diminish exhaust pipe erosion in steam turbines, as set forth in U.S. Pat. No. 4,622,819 by Silvestri and Draper, and assigned to Westinghouse Electric Corporation. A Swiss design for a moisture preseparator also discloses the problem of corrosion from the moisture content of steam leaving a high pressure turbine and is described in "Moisture Separator and Cycle Efficiency Improvements by Installing Moisture Preseparators", von Boekh, Hutton & Patrick, ASME Joint Power Generation Conference Paper No. 84-JPGC-Pwr-30, 1984.
Moisture separated from the exhaust system is typically routed first to a collection tank and then to a moisture separator reheater drain tank. A major concern with respect to the incorporation of moisture preseparators into existing nuclear steam turbine systems is the avoidance of upsets in the drain system of the separator stage in the moisture separator reheaters. It has been found that the operating pressures of the moisture preseparators, customarily located close to the turbine exhaust, are higher than those of the drain lines from the moisture separator reheaters. The pressure differences are in the neighborhood of two to three percent of turbine exhaust pressure. There is also evidence of pressure recovery of steam velocity in the preseparator water collection zone, between the preseparator inner wall and either the exhaust snout or crossunder pipe wall. This phenomenon can increase the pressure differential to four percent of turbine exhaust pressure.
Given this pressure differential of two to four percent between the preseparator drains and the moisture separator reheater (MSR) drains, a direct coupling of the preseparator drain collection tank to the moisture separator reheater drain tank could result in flashing and could impair the drainage ability of the MSR, i.e., pockets of vapor could choke the drain pipes with the increased volume of drainage fluid and thereby reduce drainage flow. The expansion and contraction of the exhaust fluid, upon formation and collapse of pockets of vapor, causes cavitation and severe mechanical stress on system hardware. When flashing occurs in the drains of the preseparator system, it can be detected by an increase in the oscillation amplitude of the liquid level of the MSR drain tank.
One approach to solving the flashing problem has been to omit insulation on portions of the drain lines from the preseparator to reduce the water temperature, and hopefully, the pressure differential at the MSR drain tank. However, this approach has not been totally effective since increased oscillation amplitude of the MSR drain tank liquid level has been observed close to the high level turbine system trip setpoint despite reduction or elimination of insulation. Furthermore, this approach has economic disadvantages, i.e., an adverse effect on plant thermal efficiency and increased load in the plant cooling and ventilation system.
In some steam turbine systems, it is not feasible to route the liquid from the preseparator drain collection tank directly to the MSR drain tank, where the MSR drain tank may be located above the preseparator collection tank. Preseparator drain water is ordinarily saturated with vapor. If it were to flow uphill, the two-to-four percent pressure differential would result in flashing. Further, MSR drain tanks may have tank level control systems, such as a valve in the MSR drain line, which would be rendered ineffective by routing the preseparator drained liquid into the MSR exit drain line. In such a configuration, an alternative would be to rout the liquid from the preseparator drain tank to a heater with a level control and connection to the heater. This would require cutting a hole in the heater shell and welding on a pipe snout. However, pressure vessel codes and NRC regulations would most probably require that such a weld be X-rayed and the heater hydrotested. These procedures are expensive and not often feasible.
It is thus an object of the present invention to provide an effective and economical method and apparatus for routing the liquid from preseparator drains to the MSR without risk of flashing and which is adapted for retrofit applications involving retrofit installation of moisture preseparator drains.